This invention relates to gas chromatographs (GCs) and more particularly to a temperature programmed GC that has a shorter time required for heating and cooling of the GC column.
Temperature programmed GCs are typically used in process applications for two reasons. The first is to provide faster analysis times for long, complex analyses. These applications are traditionally developed from laboratory techniques, where temperature programming is used routinely to speed the elution time. In this instance, the laboratory analysis is merely xe2x80x9cdroppedxe2x80x9d into the analyzer.
The second category of applications are those where the temperature programming is used to influence the desired elution. In this category are distillation analyses of mixtures with a broad boiling point range. The application of heat to these columns causes a shift in equilibrium between the gas and the liquid phase toward the gas phase which in turn causes the components to elute from the columns in a more complete and timely manner.
None of the foregoing applications are known for their speed. In some cases, the slowness of the application precludes the use of an analyzer in the control loop, relegating the analyzer to be used in an advisory capacity to the control algorithm. Therefore, the applications using these analyzers are not typically run near their limits, nor are they necessarily optimized for maximum revenue by the refiner or chemical producer.
Efforts have been made in the past to speed up the analysis times of GCs. Initial efforts were concentrated in studying the fundamentals of the partitioning process in the GC column. For example, B. O. Ayers and D. D. DeFord in xe2x80x9cHigh Speed Process Gas Chromatograph,xe2x80x9d Analytical Chemistry, 32, p 698, (1960) describe the design constraints necessary to optimize operational parameters of chromatographs to the point where a group of six hydrocarbons can be analyzed in 25 seconds.
The heart of a GC system is the column. Column performance sets limits to the separations attainable and helps determine the speed of analysis. Three basic types of columns are used in gas chromatography.
The first type of column are conventionally packed columns which have been used since the introduction of gas chromatography by A. T. James and A. J. P. Martin in 1952. By 1960, R. J. Loyd et al. reported in xe2x80x9cOptimization of Resolutionxe2x80x94Time Ratio with Packed Chromatographic Columns,xe2x80x9d Analytical Chemistry, Volume 32, Number 6, p 698 a ninefold improvement in the time required to obtain a given chromatographic separation using columns containing a low proportion of partitioning agent and a carrier gas of low viscosity and high diffusivity.
Other parameters which affect the resolution and speed of packed columns such as liquid loading, solid support characteristics, column diameter and length have been investigated and reported in numerous articles and presentations.
The second type of column is the micropacked column which is a packed column with an internal diameter of 0.5 to 1 mm and the same packing density as a conventional packed column. Micropacked columns have been used in gas chromatography since 1963. Because of the advantages micropacked columns possess, numerous process applications have been done using this column type. The advantages include reproducibility, a small carrier gas flow rate and high efficiency. Supports may be coated with any stationary phase in the desired quantity. The column packing may be prepared in large batches to ensure reproducible properties. The pressure drop is not excessive while the number of theoretical plates per unit length is high. The major problem associated with this type of column is difficulty in packing longer lengths ( greater than 10 feet). These columns are normally packed in {fraction (1/16)} inch stainless tubing and it is visually impossible to determine if there are empty spaces within the column.
The third type of column is the capillary (0.1 to 1.0 mm I.D.) or wall coated open tubular (WCOT) columns introduced by Golay in 1957. Numerous articles have been written and extensive research has been conducted to define the benefits of capillary columns in process gas chromatography for resolution and speed of analysis which these columns provide. Parameters such as column diameter/length, stationary film thickness, column material, and carrier flows/pressures have been studied and optimized for reduced analysis time in process applications. The wide-bore capillary has been of particular interest for process applications because it can be used as a direct replacement for a packed column without changing operating parameters or sample preparation. The associated benefit is a dramatic decrease in analysis time without changing sample size.
Another unique approach to providing faster analysis times without sacrificing sensitivity and requiring small sample volumes is the multicapillary column. This column was introduced by Alltech in the late 1990""s by combining over 900 liquid phase coated, 40 xcexcm capillaries in a single glass tube. Compared to conventional capillary columns, multicapillary columns maintain high efficiency across a broader flow rate range, operate at lower temperatures and provide faster analyses.
Although numerous advances have been made relative to speed of analysis by manipulating the column types and parameters, there is a theoretical limit to what can be done to decrease time for the sample to reach equilibrium between the mobile and stationary phases. Or more precisely, reduce analysis time without sacrificing separation.
Two of the factors, that affect this equilibrium time, are temperature and carrier gas pressure/flow. By increasing either or both of these parameters there will be a decrease in analysis time.
Temperature programming which is a controlled change in the temperature surrounding the column has been used to speed up the analysis time of wide boiling range samples since the early 1960""s [see for example A. J. Martin, xe2x80x9cLinear Programmed Temperature Gas Chromatography to 500xc2x0 C.,xe2x80x9d Edinburgh Symposium, London, Butterworths, 208-10 (1960)]. The most common application is the use of temperature programming for simulated distillation of fuel products. By increasing the temperature, the time spent by a sample in the liquid phase is decreased which shifts the equilibrium to the gas phase which reduces the time of analysis. One point to be considered with temperature programming is cycle times, which is the length of time from sample inject for one analysis to sample inject for the next analysis and includes cool down time. Although the analysis time may have. been significantly reduced by temperature programming, the consideration of the time it takes to cool down to the initial temperature diminishes the benefit gained. Regardless, this approach has been used to reduce the analysis time for many complex process samples and would have greater benefit if the heating and cooling cycles can be reduced.
Pressure/flow programming of the carrier gas has more recently become available on process gas chromatographs and can also be used to reduce analysis times. By increasing the pressure/flow in a controlled manner the time for the sample to reach equilibrium is reduced and the sample is swept through the column to the detector by the faster flow of the carrier gas. Pressure/flow can be used independently of temperature or both can be used simultaneously to speed the analysis. There is a special benefit to using pressure programming when the liquid phase in the column has reached its maximum operating temperature.
In the mid to late 1980""s the microchip gas chromatograph, also known as the xe2x80x9cGC on a chipxe2x80x9d, was developed and introduced by Microsensor Technology, Fremont, Calif. The major benefit associated with this development was speed of analysis which was gained through miniaturization of each of the chromatographic components, including the column which was etched on a silicon wafer. Factors such as no backflush, no liquid inject, limited column/detector choices, and lack of temperature control have limited the use of this technology to speed up applications, although recently some of the limitations have been resolved.
Another approach to speeding up of analysis cycle times has recently been introduced by Applied Automation, Inc. using a technique known as parallel chromatography. This approach has been made possible by the availability of powerful, inexpensive computerized electronic controllers. The time for the analysis to be completed is dependent on the sample train with the longest analysis time.
As was discussed above, if temperature programming of the column could be done faster from both a heating and cooling perspective, there would be significant benefits to be gained for faster process gas chromatographic analysis. This fact was recognized very early in the evolution of the technology, when in 1961 and 1963, Perkin-Elmer introduced laboratory chromatographs which used resistive or direct heating of the chromatographic column. The column is its own heating element. A low voltage, high amperage current is passed through the column, which becomes heated by resistance heating.
U.S. Pat. No. 4,726,822 describes the use of a fast response thermochromatographic capillary column which has a thin coating of a metallic compound applied to the outer surface. When a current is passed through the column it heats and cools very quickly.
Thermedics Detection, Inc., Chelmsford, Mass. has developed a very fast temperature programming technique using resistive heating of a fused silica capillary column contained within a metal tube. A current is passed through the outer tube heating the column to temperatures up to 1200xc2x0 C./min. The combination of a short column (5 mxc3x970.25 mm I.D.), a high gas flow rate (up to 10 ml/min), and fast temperature programs typically decreased analysis times from 30 minutes to about 2.5 minutes. C. Rankin and R. Sacks have reported in xe2x80x9cA Computer-Controlled, High Speed, Repetitive Gas Chromatography System,xe2x80x9d LC-GC, Volume 9, Number 6, pp 428-434, (1991) the use of a gas cooled and electrically heated metal capillary tube as a cryofocusing inlet system, and a vacuum pump for backflushing high boilers as a means to accomplish analysis cycle times in the 10-20 second range.
RVM Scientific, Inc., uses resistive heating wires wrapped around the capillary column. The wire wrapped capillary is insulated using a proprietary technique to ensure rapid and stable temperature control.
U.S. Pat. No. 5,589,630 describes a fast GC that employs low dead volume fittings, high speed injectors and detectors, a fast temperature program module, and a high speed data acquisition system. The fast temperature programming module can rapidly heat and cool the column as required to achieve analysis of compounds whose boiling points differ by as much as 250xc2x0 C. in less than two minutes, possibly in less than one minute. The fast temperature module described therein has a heating means that is preferably electrical resistance or an induction heater and uses the flow of unheated heat transfer fluid. for cooling the column.
A temperature programmed module for use in a gas chromatograph that has a micropacked column through which a current can be passed to heat the column. The module also has an inner jacket tube surrounding the column and having an outer diameter greater than the outer diameter of the column to define a space between the between the first jacket tube and the column. The module further has an outer jacket tube surrounding the inner jacket tube and having an outer diameter greater than the inner jacket tube outer diameter to define a space between the outer and the inner jacket tubes. Air having a controlled temperature flowing only into the space between the outer and the inner jacket tubes when it is desired to heat the column by passing the current through the column.
In a gas chromatograph, an enclosure whose temperature is not controlled. The enclosure has a temperature programmed module. The module has a micropacked column through which a current can be passed to heat the column. The module also has an inner jacket tube surrounding the column and having an outer diameter greater than the outer diameter of the column to define a space between the between the inner jacket tube and the column. The module further has an outer jacket tube surrounding the inner jacket tube and having an outer diameter greater than the inner jacket tube outer diameter to define a space between the outer and the inner jacket tubes. Air having a controlled temperature flowing only into the space between the outer and the inner jacket tubes when it is desired to heat the column by passing the current through the column.
A method for assembling a temperature module having a micropacked column. The method has the steps of:
(a) surrounding the column with a first jacket tube having an outer diameter greater than the outer diameter of the column to define a space between the first jacket tube and the column;
(b) surrounding the first jacket tube with a second jacket tube having an outer diameter greater than the first jacket tube outer diameter to define a space between the first and the second jacket tubes; and
(c) providing on the outer surface of the first jacket tube prior to surrounding that tube with the second jacket tube a means for centering the first jacket tube in the second jacket tube.
A method for using a temperature module having a micropacked column through which a current can be passed to heat the column. The column is surrounded by an inner jacket tube having an outer diameter which is greater than the outer diameter of the column to define a space between the column and the inner jacket tube and an outer jacket surrounding the inner jacket tube the outer jacket having an outer diameter greater than the inner jacket tube outer diameter to define a space between the outer and the inner jacket tubes. The method has the step of flowing air having a controlled temperature only into the space between the outer and the inner jacket tubes when it is desired to heat the column by passing the current through the column.
A gas chromatograph comprising:
(a) a sample injector valve;
(b) a flame ionization detector; and
(c) a temperature programmed module connected between said sample injector valve and the flame ionization detector, the module comprising:
(i) a micropacked column through which a current can be passed to heat the column;
(ii) an inner jacket tube surrounding the column and having an outer diameter greater than the outer diameter of the column to define a space between the first jacket tube and the column; and
(iii) an outer jacket tube surrounding the inner jacket tube and having an outer diameter greater than the inner jacket tube outer diameter to define a space between the outer and the inner jacket tubes;
air having a controlled temperature flowing only into the space between the outer and the inner jacket tubes when it is desired to heat the column.